1. Field of the Invention
The present invention relates to electronic systems and in particular to a protection device for buses in the electronic systems.
2. Description of the Related Art
Currently, electronic systems are becoming more and more common in today's society as the capabilities and uses of such electronic systems continue to expand. Many electronic systems are powered by batteries that can form a battery pack. The batteries can include rechargeable batteries. The rechargeable batteries can include alkaline batteries such as well-known nickel cadmium (Ni—Cd) or nickel metal hydride (Ni-MH) batteries. Recently, lithium ion (Li-ion) batteries have gained more popularity in some high-end electronic systems because they exhibit high energy density and stable storage capability.
In battery pack applications, communication between the battery pack and an external device is implemented by a bus. The bus usually is a low voltage bus, such as I2C, SMBus, etc. If the ground of the battery pack is directly connected to the ground of the external device, the bus is called a common ground bus. In this situation, two P-channel MOSFETs may be employed to control the charging and discharging of the battery.
When the ground of the battery pack is not directly connected to the ground of the external device, the bus is called a non-common ground bus. Turning to FIG. 1, a prior art battery pack application 100 with the common ground bus is illustrated. The battery pack application 100 includes a battery pack 110 and an external device 150. The battery pack 110 is composed of a battery 111, a controller 113, and a plurality of switches. In this embodiment, the plurality of switches includes two N-channel MOSFETs 115 and 117 that control the charging and discharging loops of the battery 111. The external device 150 can be a load or a charger. The battery pack 110 communicates with the external device 150 via a low voltage bus. The battery pack 110 and external device 150, each includes an embedded interface unit (not shown) that enables it to interface with the bus. Hence, the bus can also be called an interface bus.
When either the N-channel MOSFET 115 or the N-channel MOSFET 117 is turned off, the ground of the external device 150 will not be the real ground while the battery 111 is connected to the real ground. When both of the MOSFETs 115 and 117 are turned off, the ground of the external device 150 will also not be the real ground. Hence, the ground of the external device 150 will be floating. In this situation, an undesired current will flow through the bus when there is a voltage difference between the voltage VBATT of the battery 111 and the voltage VL or VCHG of the external device 150. FIG. 2A illustrates an operation mode 200A of the battery pack application 100. In this operation mode, VBATT is higher than VL or VCHG, and hence an undesired current will flow from the battery 110 to the external device 150 and then to the real ground (i.e., the ground of the battery 111) via the bus. FIG. 2B illustrates another operation mode 200B of the battery pack application 100. In this operation mode, VBATT is lower than VL or VCHG, and hence an undesired current will flow from the external device 150 to the battery 111 and then to the ground of the external device 150 via the bus. The undesired currents described above may cause the interface units to be entirely destroyed.
In order to avoid the above-mentioned problem, an isolation technology is used to isolate the communication between the battery pack 110 and the external device 150 via the bus. Turning to FIG. 3, a simplified block diagram of a prior art opto-coupler isolating application 300. In this embodiment, an isolating circuit 310 is used to isolate the bus connected to the battery pack 110 from the bus connected to the external device 150. The isolating circuit 310 can include at least one opto isolator. Although the protection for the non-common ground bus can be implemented by the isolation technique, the insertion of the opto-coupler results in power loss and extra cost to the battery pack application.
It is thus desirous to have an apparatus that provides protection for the non-common ground buses embedded in the battery pack application with low power dissipation and low cost. It is to such apparatus and method the present invention is primarily directed.